High frequency transmission line



D. R. AYER ET A].

HIGH FREQUENCY TRANSMISSION LINE July 25, 1961 2 Sheets-Sheet 1 Filed April 10, 1959 Donald R. Ayer Jesse L. Bufler Victor F. Duhlgren INVENTORS July 25, 1961 D. R. AYER ETTAL 2,994,050

HIGH FREQUENCY TRANSMISSION LINE Filed April 10, 1959 2 Sheets-Sheet 2 Fig.4

Fig.5

Donald R. Ayer Jesse L.Bufler 23 Victor F. Dohlgren INVENTORS United States Patent 2,994,050 HIGH FREQUENCY T AN SMISSION LINE Donald R. Ayer, Jesse L. Butler, and Victor F. Dahlgren, Nashua, N.H., assignors to Sanders Associates, Inc., Nashua, N.H., a corporation of Delaware Filed Apr. 10, 1959, Ser. No. 805,514 9 Claims. (Cl. 333-84) The present invention relates to transmission lines and more particularly to flexible high frequency transmission lines used in conjunction with high frequency electronic devices.

Prior art shielded high frequency transmission lines fall into three general categories-wave guides, coaxial lines and the so-called flat-strip lines. Because of their constructions each of these types of transmission lines have certain inherent deficiencies. Wave guides occupy a large amount of space, are generally rigid, are heavy, have a relatively narrow band pass characteristic and are expensive to manufacture. Coaxial transmission lines, on the other hand, are lighter, less expensive and are relatively flexible, but tend to be more leaky and more noisy than either of the other types. Furthermore, when a plurality of coaxial lines are packaged into a unit of high conductor density, much of their flexibility is lost. Probably the most undesirable feature of coaxial transmission lines, however, is their relatively high cost of manufacture as compared to strip lines, the latter type transmission line being subject to manufacture by mass production, e.g., printed circuit, techniques.

Strip transmission lines, which are rather comprehensively described in the Handbook of Tri-Plate Microwave Components, Sanders Associates, Inc., November 30, 1956, have many additional features. One of these features is that strip transmission lines allow the expression of design concepts that are impractical or even unattainable in conventional coaxial and wave-guide systems. The most complex device can be manufactured in accordance with strip line techniques as easily as the simplest. The flat design of strip transmission lines, and components for use therewith, permits fabrication directly on the dielectric medium, which, as mentioned above, is characteristic of printed circuit techniques. In addition to greatly simplifying production of microwave components, the strip transmission lines and hardware for use therewith are unusually light in weight and tend to be extremely compact. Although strip transmission lines have many other desirable characteristics their more widespread use is somewhat limited due to their lack of flexibility. Even strip lines made with flexible r thermoplastic, dielectric materials separating the inner conductor and ground planes are not useful as flexible transmission lines. Such laminates cannot be bent around a small radius because of lack of extensibility of the outer ground plane and buckling of the inner ground plane, that is to say, the ground plane on the inside of the arc. This buckling produces an impedance discontinuity which makes further use of the transmission line impractical.

It has been a problem in the past to vary the configuration of the conductors without substantially varying the characteristic impedance of the line. Variations of this character, however, normally do introduce variations in impedance or so-called impedance discontinuities. These discontinuities, in turn, tend to introduce unclesired reflections and extraneous radiation losses. The present invention is directed to an improvementin such transmission lines by providing a solution for the problems arising from attempts to achieve a high degree of flexibility while minimizing the problems of energy losses due to discontinuities along the transmission lines.

It is, therefore, an object of the present invention to provide an improved high-frequency transmission line.

It is a further object of this invention to provide an improved high-frequency transmission line exhibiting a high degree of flexibility.

An additional object of the present invention is to provide an improved flexible, high-frequency transmission line having substantially a constant characteristic impedance with minimum radiation losses.

Yet, another object of this invention is to provide an improved flexible high-frequency transmission line having a plurality of closely spaced signal energy carrying conductors exhibiting minimal coupling between the adjacent conductors.

In accordance with the present invention, there is provided a flexible high frequency transmission line. The line includes a pair of flexible, elongated, extensible ground potential outer conductors formed from a plurality of conductive elements bonded to a flexible substrate. An elongated, flexible, signal potential inner conductor is disposed in insulated spaced relation between the outer conductors. Suitable means are provided for securing the conductors in their relative positions.

For a better understanding of the present invention, together with other and further objects thereof, reference is made to the following description, taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

In the drawings:

FIG. 1 is a perspective view of an embodiment of the transmission line of the present invention;

FIG. 2 is an elevational section taken along the line 22 of FIG. 1;

FIG. 3 is a perspective view of another embodiment of the transmission line of the present invention;

FIG. 4 is an elevational section taken along the line 4-4 of FIG. 3;

FIG. 5 is a perspective view of yet another embodiment of the transmission line of the present invention; and

FIG. 6 is an elevational section taken along the line 66 of FIG. 5.

Referring now to the drawings and with particular reference to FIGS. 1 and 2, there is here illustrated a flexible transmission line 11 embodying the present invention. The line has a pair of flexible, elongated, extensible ground potential outer conductors 12 providing ground planes. A plurality of elongated, flexible, signal, planar inner conductors 13 are disposed as shown, between the outer conductors 12. The inner conductors 13 are narrower than the outer conductors 12 and each inner conductor 13 is preferably equidistant from the outer conductors 12. The outer conductors 12 are formed from a plurality of overlapping conductive elements bonded to a flexible substrate 14 as for example one of the foamed, synthetic, organic plastics. The flexible substrate 14 may be of varying density in order to achieve a desired value over the range of dielectric constants possible from. the use of a given plastic. The substrate 14 additionally serves as a means for securing the conductors in their relative positions. In order to maintain the proper lateral spacing of the inner conductors 13 a less porous insulating material 15 may be used as a base or to fully encapsulate the inner conductors 13 prior to the lamination of the inner and outer conductors 13 and 12 into an integral transmission line.

Although the overlapped areas of the outer conductors 12 in this embodiment tend to appear as impedance discontinuities when this overlapped area is large relative to a wave length, this problem becomes insignificant when the overlapped area is maintained below A wave length at the highest operating frequency of the line. This feature is more particularly illustrated in FIG. 1.

One of the unique features of the transmission line of FIG. 1 is the lapped or shingled ground planes.

Effectively, the spacing between the outer conductors is periodically varied due to the overlapping elements. This is a radical departure from any of the teachings in the strip line art. Because of the inherent balance of the ideal strip line configuration the fields above and below a central plane through the line are equal and opposite. No parallel plate TEM, or TE zero-order modes exist as long as the symmetry of such structure is maintained. In the ordinary flat strip line, however, longitudinal tilting of the center strip between the ground planes excites higher order modes. Tilting may arise under pressure or any condition which separates the ground planes. In view of such teachings of the prior art, it appears obvious that any kink, indentation, or variation in the spacing of the ground planes would produce a serious impedance discontinuity. In the instant embodiment, however, as mentioned above the overlapping of the conductive elements of the ground planes or outer conductors 12, do not, in fact, appear as impedance discontinuities because of the relatively small area of the lap.

Another unique feature of this embodiment is the charactor of the overlapped conductive elements and the substrate 14 to which they are bonded. The overlapping builds a certain amount of slack into the outer conductors. Here the cross-section of the transmission line 11 is loosened up by employing a compressible, elastic, p orous insulating material for the substrate 14 to occupy the space between the inner and outer conductors 13 and 12. This makes it possible for stretching to occur in the outer layer around the bend and compression to occur in the inner layer. Unless this can occur, serious changes in the electrical properties accompany any deformation of the line since the center conductor tends to crush the dielectric and move out next to the ground plane on the outside of the bend, while the inner ground plane buckles and separates widely from the center conductor. With individual conductive elements for example a /2" in width, the total deformation at each overlap would not exceed about 5 while bending the cable around a 6" radius, i.e. the angles of the polygon formed by wrapping the cable around the cylnder of 6" radius would be less than 5. The overall thickness of the transmission line 11 must also be controlled, since it should not exceed /2 wave length at the highest operating frequency of the line to avoid wave guide modes of propagation. This limitation is indicated in FIG. 2.

Referring now to- FIGS. 3 and 4 of the drawings, there is here illustrated another embodiment 16 of the transmission line of the present invention. In this embodiment the outer conductors are formed from a plurality of inner segments 17 and a plurality of outer segments 18 which are superimposed upon the gaps between the inner segments 17. These inner and outer segments are maintained in position by a flexible substrate 19 which also serves to encapsulate the inner conductors 20 and maintain them in spaced relation with respect to each other and with respect to the outer conductors.

As in the embodiment of FIGS. 1 and 2 this embodiment is characterized by the fact that the spacing between the outer conductors is periodically varied. The overlap area of the inner and outer ground plane segments should likewise be maintained below A wave length at the highest operating frequency of the line, as more particularly illustrated in FIG. 3. In addition to providing predetermined points at which the transmission line will flex, the overlapped segments also serve to provide extra conductor length for a given length of transmission line, thereby permitting conductors on the outside of the bend to stretch. This inhibits sharp buckling at any point.

The embodiment of FIGS. 3 and 4 is particularly useful in minimizing losses due to radiation as, even when flexed, there are no apertures in the ground plane which would permit radiation.

Illustrated in FIGS. 5 and 6 of the transmission line of the is yet another embodiment present invention. There is here shown a transmission line having a pair of flexible, elongated, extensible ground potential outer conductors 21 formed from a plurality of U-shaped conductive elements 22. The legs 23 of the U-shaped elements 22 are preferably shorter than A wave length at the highest operating frequency of the line to minimize radiation. This limitation is particularly illustrated in FIG. 5.

Although radiation losses will occur in the transmission line of this embodiment at high frequencies, this detriment is somewhat counterbalanced by its extreme flexibility. As mentioned above, when the transmission line is flexed, a strain is placed upon all material at the bend except that portion of the line lying on the neutral axis of the bend. In other words, as a transmission line is flexed, the material on the inside of the bend is compressed and that on the outside of the bend is stretched. Somewhere between the areas of compression and stretching is the neutral axis which is tree of distortional force. In order to attain a high degree of flexibility, a transmission line must be made of material and be of a design which will adequately withstand both compression and stretching with a minimum of permanent distortion. This embodiment of the present invention, when fabricated from the proper materials, will possess the desired high degree of flexibility.

While applicant does not intend to be limited to the use of any particular materials in the manufacture of the transmission lines of the present invention, the combination of copper conductors and foamed polyurethane insulation has been found to be particularly useful. For example, in the transmission line of FIG. 1 the ground plane conductors may be 2-ounce copper (0.0027 inch thick) and sufliciently wide to extend laterally the outer most inner conductors. Overlap of adjacent segments may be, for example, 0.250 inch. The inner conductors 13 are, for example, 0.008 inch thick by 0.025 inch wide and are spaced, for example, /8 inch apart. To improve the appearance of the transmission line of the present invention and to secure a better bond to the plastic insulating material, the copper conductors may have a black cupric oxide coating. This coating may be produced anodically or by means of a chemical bath. Such processes are fully described in the Meyer US. Patent No. 2,364,993 and Hurd US. Patent No. 2,828,250. Other plastic materials that have successfully been employed to improve the article of this invention include foam vinyl and elastomers such as neoprene or silicone rubber. It is believed, however, that this principle applies broadly to many plastics and conducting materials and applicants do not intend to be limited to those cited in the examples.

A prefer-red method of manufacturing the transmission lines of the present invention is by the use of a threesided mold and a foam-in-place plastic insulating materials. For example, molds are readily available that can be adapted for such a process by slitting the side walls to receive the outer conductor segments and to maintain them in their proper positions. The inner conductors are then clamped between opposite ends of the mold Where they are readily maintained in proper spacing. The insulation is poured into the mold and permitted to foam into place and out of the open top of the mold. Any excess insulation foaming out of the mold is readily trimmed off flush with the mold edge, thus providing a transmission line of the desired dimensions. Foam-inplace resins, such as polyurethane, readily adhere to cupric-oxide coated copper conductors or other properly prepared surfaces.

An alternative method of making the transmission line of the. present invention involves bonding the inner and outer conductors to thin thermoplastic sheets which act as carriers for the conductors and which maintain the conductor segments in the desired relationship with one another. The conductors in the carrier sheets are in turn bonded to pre-cut strips of foamed insulation under heat and pressure to form the finished cable.

The present invention presents an important step forward'in the art of transmission lines in that the dielectric and flexibility properties of various plastic materials may be successfully utilized to achieve a heretofore unrealized result in the manufacture of transmission lines.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A flexible, high frequency transmission line, comprising: a pair of flexible, elongated, extensible substantially planar ground potential outer conductors each formed from a plurality of conductive elements bonded in longitudinally overlapping relation to a flexible substrate of dielectric material; an elongated, flexible, signal potential inner conductor disposed in insulated spaced relation between said outer conductors; said substrate securing said conductors in their relative positions.

2. A flexible, high frequency transmission line, comprising: a pair of flexible, elongated, extensible substantially planar ground potential outer conductors each formed from a plurality of conductive elements bonded in longitudinally overlapping relation to a flexible substrate of dielectric material; elongated, flexible, signal potential inner conductors disposed in insulated spaced relation between said outer conductors; and resilient, flexible insulating material to provide means for securing said inner conductors in their relative positions.

3. A flexible, high frequency transmission line, comprising: a pair of flexible, elongated, extensible substantially planar ground potential outer conductors each formed from a plurality of overlapping conductive elements bonded to a flexible substrate of dielectric material; said conductive elements being in insulated spaced relation less than a A Wave length apart at the highest operating frequency of the line, elongated, flexible, signal potential inner conductors disposed in insulated spaced relation between said outer conductors; said substrate securing said inner conductors in their relative positions.

4. A flexible, high frequency transmission line, comprising: a pair of flexible, elongated, extensible substantially planar ground potential outer conductors formed from a plurality of conductive elements bonded in longitudinally overlapping relations to a flexible substrate of dielectric material; an elongated, flexible, signal potential inner conductor, narrower than said outer conductors, disposed in insulated spaced relation between said outer conductors; and means for securing said conductors in their relative positions.

5. A flexible, high frequency transmission line, comprising: a pair of flexible, elongated, extensible substantially planar ground potential outer conductors each formed from a plurality of overlapping conductive elements bonded to a flexible substrate of dielectric material, said elements overlapping less than one quarter wave length at the highest operating frequency of said line;

an elongated, flexible, signal potential inner conductor disposed in insulated spaced relation between said outer conductors; said substrate securing said conductors in their relative positions.

6. A flexible, high frequency transmission line, comprising: a pair of flexible, elongated, extensible substantially planar ground potential outer conductors each formed from a plurality of conductive elements bonded in longitudinally overlapping relation to a flexible substrate of dielectric material; elongated, flexible, signal potential inner conductor, narrower than said outer conductors, disposed between said outer conductors in insulated spaced relation less than one-quarter wave length therefrom at the highest operating frequency of said line; and means for securing said inner conductors in their relative positions.

7. A flexible, high frequency transmission line, comprising: a pair of flexible, elongated, extensible substantially planar ground potential outer conductors formed from a plurality of spaced conductive elements bonded to a flexible substrate of dielectric material and a second plurality of conductive elements superimposed on the spaces between said first plurality of conductive elements and also bonded to said substrate; an elongated, flexible, signal potential inner conductor, disposed in insulated spaced relation between said outer conductors; said substrate securing said conductors in their relative positions.

8. A flexible, high frequency transmission line, comprising: a pair of flexible, elongated, extensible ground potential outer conductors each formed from a plurality of U-shaped conductive elements bonded to a flexible substrate of dielectric material; an elongated, flexible, signal potential inner conductor disposed in insulated spaced relation between said outer conductors, the legs of said U-shaped elements extending away from said inner conductors and the intermediate portions of said U- shaped elements being substantially planar; said substrate securing said conductors in their relative positions.

9. A flexible, high frequency transmission line, comprising: a pair of flexible, elongated, extensible ground potential outer conductors each formed from a plurality of spaced U-shaped conductive elements bonded to a flexible subtrate of dielectric material, the legs of said U- shaped elements being shorter than one-quarter wave length at the highest operating frequency of said line; elongated, flexible, signal potential inner conductors disposed in insulated spaced relation between said outer conductors, the legs of said U-shaped elements extending away from said inner conductors and the intermediate portions of said U-shaped elements being substantially planar; and means for securing said inner conductors in their relative positions.

References Cited in the file of this patent UNITED STATES PATENTS 2,178,299 Dallenbach Oct. 31, 1939 2,759,990 Bean Aug. 21, 1956 FOREIGN PATENTS 453,182 Canada Dec. 7, 1948 708,601 Great Britain May 5, 1954 767,067 Great Britain Jan. 30, 1957 

